1. Field of the Invention
The present invention relates to an image forming apparatus employing two different image forming sections in combination, of which one is designed for performing monochromatic image formation based on a method using a developer, such as an electrophotographic system, and the other is designed for performing color image formation based on a method using ink, such as an ink-jet system.
2. Description of the Related Art
Conventionally, as image forming methods for forming a color image, there have been known a method using a developer, such as an electrophotographic system, and a method using ink, such as an ink-jet system. These two conventional systems each have advantage and disadvantage. Regarding the electrophotographic system, the positive side is that speeding-up of image formation can be achieved by forcibly fixing a developer onto a paper sheet through application of heat and pressure, and that the running cost can be reduced by using a relatively inexpensive developer. The negative side is that, since a developer in use, which is obtained by mixing pigment or dye into powder of thermoplastic resin, is transferred onto recording paper and is then fixed thereon by heating, a change of hue is likely to occur due to the influence of the transparency of the resin powder and to a temperature rise occurring during the fixation, resulting in poor color reproducibility. Likewise, regarding the ink-jet system, the positive side is, that image formation is achieved by using highly-transparent liquid ink without performing heating at high temperature, resulting in excellent color reproducibility. The negative side is that much time needs to be spent in drying the ink and thus speeding-up of operation cannot be achieved, and that the running cost is sharply increased because of the use of relatively expensive ink.
Moreover, in general, as data on color images to be formed onto recording paper, rather than full-color image data, partial-color image data is used more frequently that corresponds to a seal, illustration, or graph represented in a single color or a plurality of colors, which is included in part of text images of a document or the like.
With consideration given to the characteristics of each of the above-described image forming methods and to color image usage conditions, there has conventionally been proposed an image forming apparatus employing two different image forming sections in combination, of which one is designed for performing image formation based on a method using a developer, such as an electrophotographic system, and the other is designed for performing image formation based on a method using ink, such as an ink-jet system. In this construction, monochromatic image formation is performed in the developer-system image forming section, whereas color image formation is performed in the ink-system image forming section.
For example, Japanese Unexamined Patent Publication JP-A 8-95463 (1996) discloses an image forming apparatus which is so designed that, after completion of electrophotographic system-based image formation, ink-jet system-based image formation is performed. This construction is characterized in that means for changing the speed of conveyance of recording paper, or means for cooling recording paper is separately provided for each of the following individual cases: a case of performing image formation based solely on the electrophotographic system; a case of performing image formation based on both of the electrophotographic system and the ink-jet system; and a case of performing image formation based solely on the ink-jet system. In this arrangement, the temperature of the recording paper, which has passed through a fixation unit incorporated in the electrophotographic-system image forming section, can be kept within a predetermined range at all times, whereby making it possible to prevent deterioration in image quality due to ink blot produced during the ink-jet-system image formation.
However, in the conventional image forming apparatus employing a developer-system image forming section and an ink-system image forming section in combination, it has proved impossible to make sufficiently short the paper conveying path running through the developer-system image forming section and the ink-system image forming section, in consideration of the difference in image forming speed between the developer system and the ink system. Consequently, the apparatus as a whole cannot be kept satisfactorily compact.
That is, in general, the developer system and the ink system differ from each other in paper conveyance speed for image formation. Thus, in a case where the distance between an upstream-side image forming section and a downstream-side image forming section is set to be shorter than the length of recording paper, before the rear end of the recording paper passes through the upstream-side image forming section, the front end of the recording paper reaches the downstream-side image forming section. As a result, the recording paper suffers from slack or is pulled forward and backward, causing paper jamming or tearing.
Moreover, in the construction in which the distance between the upstream-side image forming section and the downstream-side image forming section is set to be shorter than the length of recording paper, assume that the ink-system image forming section arranged downstream of the developer-system image forming section is provided with an ink head which is moved reciprocally in a main scanning direction orthogonal to the paper conveying direction, and that conveyance of recording paper is brought to a stop during the movement of the ink head in the main scanning direction. In this case, in the upstream-side developer-system image forming section, even during the interval when no image formation is being performed, it is impossible to let recording paper to pass through, at uniform speed, the fixation section maintained at a predetermined high temperature. This leads to unevenness in the degree of influence exerted upon the recording paper as a result of heating and pressurizing performed by the fixation section, giving rise to lack of uniformity in the image forming condition.
By contrast, in the construction disclosed in Japanese Unexamined Patent Publication JP-A 10-10819 (1998), arranged on the downstream side of the electrophotographic-system image forming section along the paper conveying direction is the ink-jet-system image forming section, and the conveying path extending from a fixation roller included in the electrophotographic-system image forming section to an ink jet head included in the ink-jet-system image forming section is made longer in length than recording paper. In this construction, the paper conveying path is so long that the apparatus as a whole needs to be made unduly large.
Moreover, the conventional image forming apparatus employing a developer-system image forming section and an ink-system image forming section in combination pays no regard to the case of forming images on both surfaces of recording paper, and thus the following problem is posed. In each of the case of forming any of a monochromatic image, a color image, and a mixed image obtained by combining monochromatic and color images, on only one surface of recording paper, and the case of forming any of the above-mentioned images on both surfaces of the recording paper, it is impossible to supply image data to each of the image forming sections in a manner suited for the conveying path which is made shortest in length so as to achieve the highest image formation speed. This leads to poor color reproducibility.
Further, the developer-system image forming section is typically provided with a fixation unit for applying heat and pressure to recording paper, so as to fix a developer image onto recording paper. In this case, recording paper undergoes deformation due to heating and pressurizing performed by the fixation unit. Accordingly, in order to form a mixed image, which is obtained by combining monochromatic and color images, on one surface or both surfaces of the recording paper, image formation is inevitably performed on the recording paper having been deformed due to heating and pressurizing. In this respect, since the conventional image forming apparatus pays no regard to adjustment of quantity of image data, there appears disconformity in the relative size between monochromatic and color images formed onto recording paper. This leads to poor color reproducibility.